Novel Silicon Phases and Nanostructures for Solar Energy Conversion

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1 Novel Silicon Phases and Nanostructures for Solar Energy Conversion Stefan Wippermann, E. Scalise, M. Vörös, D. Rocca, A. Gali, F. Gygi, G. Zimanyi, G. Galli E-MRS Spring Meeting, NanoMatFutur research group 13N12972

2 3rd generation solar cells: MEG + IB O. Semonin et al., SPIE Newsroom (2012) multi-exciton generation intermediate band 0.9 ev? 1.5 ev 2.4 ev A. J. Nozik, Physica E 14, 115 (2002) B. R. D. Schaller, V. I. Klimov, PRL 92, (2004) A. Luque and A. Martí, B. PRL 78, 5014 (1997) 2

3 Outline Isolated Silicon nanoparticles Electronic properties of Si NPs: optical gaps and carrier multiplication intermediate band solar cells: ideas and work in progress Embedded nanoparticles and assemblies NPs in chalcogenide matrices: charge extraction for solar energy conversion 3

4 Predictive Theory and Simulations Obtain integrated understanding enable rational device design: spectroscopy structure Structure Function Theory Experiment electronic and transport properties interface chemistry and assembly 4

5 Predictive Theory and Simulations Obtain integrated understanding enable rational device design: spectroscopy structure Structure Function Theory Experiment electronic and transport properties What can we learn from first principles calculations? interface chemistry and assembly electronic structure calculations from density functional theory (DFT) and many body perturbation theory (MBPT) ab initio molecular dynamics (MD) 4

6 Optical properties of nanoparticles in vacuo optical gap ideal /crystalline size-dependent optical gaps: quantum confinement quantum confinement enables efficient multi-exciton generation modified bulk Si nanoparticle size 5

7 Optical properties of nanoparticles in vacuo optical gap ideal /crystalline size-dependent optical gaps: quantum confinement quantum confinement enables efficient multi-exciton generation various modifications cause (mostly) red-shift in optical absorption modified bulk Si nanoparticle size surface chemistry (e. g. oxygen absorption, ligands) surface reconstruction Key role of surface structure, ligands, strain quantum confinement is only one element of the quantum dot story A. Puzder et al., PRL 88, (2002) A. Puzder et al., PRL 91, (2003) E. Draeger et al., PRL 90, (2003) F. Reboredo et al., JACS 125, (2003) L. Dal Negro et al., APL 88, (2006) SW, Y. He, M. Vörös, G. Galli, Appl. Phys. Rev. 3, (2016) 5

8 Optical properties of nanoparticles in vacuo optical gap ideal /crystalline size-dependent optical gaps: quantum confinement quantum confinement enables efficient multi-exciton generation various modifications cause (mostly) red-shift in optical absorption modified bulk Si nanoparticle size surface chemistry (e. g. oxygen absorption, ligands) surface reconstruction Key role of surface structure, ligands, strain novel core structures novel properties? quantum confinement is only one element of the quantum dot story A. Puzder et al., PRL 88, (2002) A. Puzder et al., PRL 91, (2003) E. Draeger et al., PRL 90, (2003) F. Reboredo et al., JACS 125, (2003) L. Dal Negro et al., APL 88, (2006) SW, Y. He, M. Vörös, G. Galli, Appl. Phys. Rev. 3, (2016) 5

9 SW, Y. He, M. Vörös, G. Galli, Appl. Phys. Rev. 3, (2016)

10 Optical properties of high pressure Si nanoparticles high pressure silicon cores can we design new properties, different from those of quantum dots with diamond cores? Si BC8: 0.03 ev direct gap H. Zhang et al., PRL 118, (2017) Si34H36 (R8) Si34H38 (BC8) Si42H48 (Ibam) Si35H36 (cd) Si39H40 (hd) Si46H52 (ST12) Si44H48 (bct)

11 Find Si NPs with solar gaps and efficient MEG high pressure Si cores lower the gap and increase multi-exciton probability E qp n = E n + h n ˆ (E qp n ) ni h n ˆV XC ni electronic gap Si123H100 Si144H ev 1.8 ev Si35H36 (cd) 1.1 ev 0.5 ev SW, M. Vörös, D. Rocca, A. Gali, G. Zimanyi, G. Galli, Phys. Rev. Lett. 110, (2013) Si34H38 (BC8) 8

Find Si NPs with solar gaps and efficient MEG high pressure Si cores lower the gap and increase multi-exciton probability II i = 2 ~ MEG rate X f hx i W XX f i 2 (E i E f ) Si NPs with BC8 cores

12 Find Si NPs with solar gaps and efficient MEG high pressure Si cores lower the gap and increase multi-exciton probability II i = 2 ~ MEG rate X f hx i W XX f i 2 (E i E f ) Si NPs with BC8 cores exhibit efficient MEG within solar spectrum Si123H100 Si144H ev 1.1 ev 1.8 ev 0.5 ev Si35H36 (cd) SW, M. Vörös, D. Rocca, A. Gali, G. Zimanyi, G. Galli, Phys. Rev. Lett. 110, (2013) Si34H38 (BC8) 9

Germanium NPs with exotic core structures colloidal Si-BC8 and

techniques, no need for high pressures Ge NPs with ST12 cores also

, JACS 136, 1296 (2014) S. J. Kim et al., J. Mater. Chem.

13 Germanium NPs with exotic core structures colloidal Si-BC8 and Ge-ST12 nanoparticles can both be synthesized by wet chemical techniques, no need for high pressures Ge NPs with ST12 cores also exhibit strongly enhanced MEG rates S. Ganguly et al., JACS 136, 1296 (2014) S. J. Kim et al., J. Mater. Chem. 20, 331 (2010) M. Vörös, SW, B. Somogyi, A. Gali, D. Rocca, G. Galli, G. Zimanyi, J. Mat. Chem. A 2, 9820 (2014) 10

14 Outline Isolated Silicon nanoparticles Electronic properties of Si NPs: optical gaps and carrier multiplication intermediate band solar cells: ideas and work in progress Embedded nanoparticles and assemblies NPs in chalcogenide matrices: charge extraction for solar energy conversion 11

15 3rd generation solar cells: intermediate bands optimal band gap of 2.4 ev and an intermediate band (IB) intermediate band partially filled to make upconversion a linear process formation of an intermediate band with finite width may help reduce unwanted non-radiative recombination existing realizations: intra-gap impurity states in bulk materials epitaxial quantum dots 0.9 ev 1.5 ev 2.4 ev A. Luque and A. Martí, PRL 78, 5014 (1997) 12

band with finite width may help reduce unwanted non-radiative recombination existing realizations: intra-gap impurity states in bulk

16 3rd generation solar cells: intermediate bands optimal band gap of 2.4 ev and an intermediate band (IB) intermediate band partially filled to make upconversion a linear process formation of an intermediate band with finite width may help reduce unwanted non-radiative recombination existing realizations: intra-gap impurity states in bulk materials epitaxial quantum dots design optimal material with intermediate bands implement intermediate band paradigm in colloidal nanoparticle solar cells 0.9 ev 1.5 ev 2.4 ev A. Luque and A. Martí, PRL 78, 5014 (1997) 12

17 Intermediate band formation in core/shell nanoparticles combine 2 materials with (almost) zero band offset but significantly different electronic gaps small core large separation between core states core states located inside electronic gap of the shell C. Cirlugano et al., Nat. Comm. 5, 4148 (2014) N. Makarov et al., ACS Nano 10, (2016) 13

Cirlugano et al., Nat. Comm. 5, 4148 (2014) N. Makarov et al.

18 Intermediate band formation in core/shell nanoparticles combine 2 materials with (almost) zero band offset but significantly different electronic gaps small core large separation between core states core states located inside electronic gap of the shell C. Cirlugano et al., Nat. Comm. 5, 4148 (2014) N. Makarov et al., ACS Nano 10, (2016) optical excitation of shell strongly asymmetric distribution of electron and hole energies exceptionally high multi-exciton rates and low threshold also upconversion demonstrated 13

Ratchet states to prevent recombination finite width of intermediate band to reduce unwanted recombination formation of ratchet states similar to 3- or 4-level systems used in lasers e. g.

19 Ratchet states to prevent recombination finite width of intermediate band to reduce unwanted recombination formation of ratchet states similar to 3- or 4-level systems used in lasers e. g. k-selection rules have been successfully used in Ge quantum dots for upconversion bulk-ge conduction band minimum at L slightly lower than direct transition at Γ HOMO-LUMO transition in Ge-NPs forms sufficiently long-lived excited state M. Wistey et al., PVSC (2014) IEEE 40th 14

20 O currently exploring epitaxial core/shell NPs and superlattices for intermediate band formation necessary system size (e. g. Sn35/Si267) is comparatively large for many body perturbation theory calculations some work on our code needed -1 GaN GaN/InN include additional multi-exciton generation pathways available due to intermediate states -2 multi-exciton generation + intermediate band E! Evac (ev) Si Si/Sn -7 CdTe CdTe/HgTe 15

21 Outline Isolated Silicon nanoparticles Electronic properties of Si NPs: optical gaps and carrier multiplication intermediate band solar cells: ideas and work in progress Embedded nanoparticles and assemblies NPs in chalcogenide matrices: charge extraction for solar energy conversion 16

22 Quantum dots live in complex environments develop structural models taking into account realistic and complex environments embedded nanoparticles assemblies of nanoparticles 17

Key role of nanoparticle surface chemistry Solution-based synthesis (N2 H5 )4 Sn2

H4 + 2H2 S solid nanocomposites: buried nano-interfaces atomistic details at

, Nano Today 11, 98 (2016) large surface-to-volume ratio NP-matrix interfaces

sensitive to nature of surface ligands M. Kovalenko et al.

, Science 324, 1417 (2009), JACS 134, 18585 (2012), Nature Mat.

23 Key role of nanoparticle surface chemistry Solution-based synthesis (N2 H5 )4 Sn2 S6 ligand exchange: inorganic ligands + heat treatment organic ligands SnS2 + 4N2 H4 + 2H2 S solid nanocomposites: buried nano-interfaces atomistic details at interface X. Ji et al., Nano Today 11, 98 (2016) large surface-to-volume ratio NP-matrix interfaces determine properties of composite optical and electronic properties are highly sensitive to nature of surface ligands M. Kovalenko et al., ACS Nano 9, 1012 (2015) D. Talapin et al., Science 324, 1417 (2009), JACS 134, (2012), Nature Mat. 12, 410 (2013) Nature Mat. 15, 142 (2016) Science 353, 6302 (2016) contrary to organic ligands, absorbed as intact units, inorganic ligands may dissociate on the NC surface inorganic ligands can form conductive glue between NPs 18

Strategy to explore buried nanointerfaces

solids complex NC/matrix interface complex

secondary reactions inside matrix: formation of

Validation by experiments: XPS, Raman, etc.

24 Strategy to explore buried nanointerfaces Structural and electronic properties of nanocrystal solids complex NC/matrix interface complex (amorphous) matrix: stochiometry, density, defects secondary reactions inside matrix: formation of H2S, Sn3As4 and As2S3 Finite temperature simulations interpret experiments: e.g. negative photoresponse, high electron mobility Validation by experiments: XPS, Raman, etc. ab-initio thermodynamics study of extended crystal surface to predict atomistic details at NC interface 19

incorporation into subsurface layers intact Sn2S6 units absorbed on S-passivated surfaces, but still unstable

25 Ab initio thermodynamics energy differences on the order of a few mev/å statistical distribution of surface structural details thermodynamic driving force towards ligand dissociation and S passivation of InAs surface sulfur incorporation into subsurface layers intact Sn2S6 units absorbed on S-passivated surfaces, but still unstable towards tetrahedral SnS4 units globally most stable structural motifs consist of passivated NC-surfaces in amorphous SnSx matrix 20

26 Validation of interface structures angle resolved XPS to characterize the surface of InAs(111) wafers treated with (NH4)2S integrated peak area of As signal suggests homogenous distribution of As in sample depth non-linear decrease of S signal indicates higher concentration of S slightly below surface broadening of the Raman features suggests lattice disorder in the nanocrystal composite 21

Joint computational synthesis and characterization from ab initio MD ab initio MD: insert Si nanoparticle in crystalline ZnS; amorphize ZnS matrix in repeated annealing cycles; remove small Zn

27 Joint computational synthesis and characterization from ab initio MD ab initio MD: insert Si nanoparticle in crystalline ZnS; amorphize ZnS matrix in repeated annealing cycles; remove small Zn clusters S is drawn from the matrix and the Si surface is terminated by sulfur the electronic gap of the NP is substantially lowered ZnS CBM Si NP LUMO ZnS VBM Si NP HOMO engineer S content to form type-ii heterojunction NP-HOMO Si-NPs in vacuum Si-NPs in ZnS Si-NPs in SiO2 sulfur lone pairs SW, M. Vörös, A. Gali, G. Zimanyi, G. Galli, Phys. Rev. Lett. 112, (2014) 22

28 Joint computational synthesis and characterization from ab initio MD computational spectroscopy: verify band offsets and band gaps found at PBE level of theory with hybrid functionals and GW calculations Solar nanocomposites with complementary charge extraction pathways for electrons and holes: Si embedded in ZnS valence band edge SW, M. Vörös, A. Gali, G. Zimanyi, G. Galli, Phys. Rev. Lett. 112, (2014) conduction band edge 23

29 Complex structural details at interface and matrix Sn106S121 subsurf. S Sn143S158 Sn88S191 Sn106S227 SnS Sn77S231 Sn88S264 SnS2 Sn215S230 SnS3 Sn106S333 Sn143S301 24

30 Complex structural details at interface and matrix nanopores Sn106S121 subsurf. S Sn143S158 Sn88S191 Sn106S227 SnS Sn77S231 Sn88S264 SnS2 Sn215S230 SnS3 Sn106S333 Sn143S301 24

31 Complex structural details at interface and matrix nanopores Sn106S121 subsurf. S Sn143S158 Sn88S191 Sn106S227 SnS Sn77S231 Sn88S264 SnS2 Sn215S230 SnS3 Sn106S333 Sn143S301 subsurface S + matrix-as 24

32 Complex structural details at interface and matrix nanopores Sn106S121 subsurf. S Sn143S158 Sn88S191 Sn106S227 SnS Sn77S231 Sn88S264 SnS2 Sn215S230 SnS3 Sn106S333 Sn143S301 subsurface S + matrix-as sulfur chains 24

Negative photoconductivity Stöckmann model: (I) hot electrons trapped in DSS (II) short conductivity boost by photogenerated holes before trapped in originally filled ASS (III) mobile electron

33 Negative photoconductivity Stöckmann model: (I) hot electrons trapped in DSS (II) short conductivity boost by photogenerated holes before trapped in originally filled ASS (III) mobile electron recombines with hole in ASS (IV) electron in DSS either relaxes into conduction band or undergoes nonradiative recombination negative photoresponse of sulfide-embedded InAs-NP composite observed in experiment W. Liu, J.-S. Lee, D. Talapin, JACS 135, 1349 (2013) 25

34 Negative photoconductivity Energy Stöckmann model: CBM ev NP-surface (I) hot electrons trapped in DSS (II) short conductivity boost by photogenerated holes before trapped in originally filled ASS CBM (III) mobile electron recombines with hole in ASS (IV) electron in DSS either relaxes into conduction band or undergoes nonradiative recombination EFermi 1.5 ev VBM 0 Distance to NP-center W. Liu, J.-S. Lee, D. Talapin, JACS 135, 1349 (2013) 26

35 Negative photoconductivity Energy Stöckmann model: CBM ev NP-surface (I) hot electrons trapped in DSS (II) short conductivity boost by photogenerated holes before trapped in originally filled ASS CBM (III) mobile electron recombines with hole in ASS (IV) electron in DSS either relaxes into conduction band or undergoes nonradiative recombination EFermi 1.5 ev VBM subsurface sulfur 0 Distance to NP-center W. Liu, J.-S. Lee, D. Talapin, JACS 135, 1349 (2013) 26

36 Negative photoconductivity Energy Dss Stöckmann model: CBM ev NP-surface As in Matrix (I) hot electrons trapped in DSS (II) short conductivity boost by photogenerated holes before trapped in originally filled ASS CBM (III) mobile electron recombines with hole in ASS (IV) electron in DSS either relaxes into conduction band or undergoes nonradiative recombination EFermi 1.5 ev VBM subsurface sulfur 0 Distance to NP-center W. Liu, J.-S. Lee, D. Talapin, JACS 135, 1349 (2013) 26

37 Negative photoconductivity Energy Stöckmann model: Dss As in Matrix CBM ev (II) short conductivity boost by photogenerated holes before trapped in originally filled ASS NP-surface (III) mobile electron recombines with hole in ASS CBM Ass EFermi (I) hot electrons trapped in DSS 1.5 ev (IV) electron in DSS either relaxes into conduction band or undergoes nonradiative recombination Sn undercoord. VBM subsurface sulfur 0 Distance to NP-center W. Liu, J.-S. Lee, D. Talapin, JACS 135, 1349 (2013) 26

38 Negative photoconductivity Energy Stöckmann model: Dss As in Matrix CBM ev NP-surface Sn overcoord. (II) short conductivity boost by photogenerated holes before trapped in originally filled ASS (III) mobile electron recombines with hole in ASS CBM Ass EFermi (I) hot electrons trapped in DSS 1.5 ev (IV) electron in DSS either relaxes into conduction band or undergoes nonradiative recombination Sn undercoord. VBM subsurface sulfur 0 Distance to NP-center W. Liu, J.-S. Lee, D. Talapin, JACS 135, 1349 (2013) 26

Negative photoconductivity Energy Stöckmann model: Dss As in Matrix CBM+1 0.5 ev NP-surface Sn overcoord.

hole in ASS CBM Ass EFermi (I) hot electrons trapped in DSS 1.

39 Negative photoconductivity Energy Stöckmann model: Dss As in Matrix CBM ev NP-surface Sn overcoord. (II) short conductivity boost by photogenerated holes before trapped in originally filled ASS (III) mobile electron recombines with hole in ASS CBM Ass EFermi (I) hot electrons trapped in DSS 1.5 ev (IV) electron in DSS either relaxes into conduction band or undergoes nonradiative recombination Sn undercoord. sulfur chains VBM subsurface sulfur 0 Distance to NP-center W. Liu, J.-S. Lee, D. Talapin, JACS 135, 1349 (2013) 26

40 Summary and conclusions Knobs and concepts to design silicon with target properties high pressure and exotic Si core structures at the nanoscale core/shell structures embedding and assembling NPs control of matrix density, applied strain, stoichiometry, interface chemistry and defects 27

41 Acknowledgements Collaborating Groups Theory: Giulia Galli (U Chicago) Francois Gygi (UC Davis) Gergely Zimanyi (UC Davis) Adam Gali (Budapest Univ.) Experiment: Dmitri Talapin (U Chicago) Gerd Bacher (U Duisburg-Essen) Sue Kauzlarich (UC Davis) Sue Carter (UC Santa Cruz) Marton Vörös Dario Rocca Emilio Scalise Taufik Adi Nugraha Lei Yang Abdus Samad Musa Alaydrus Ulrich Biedermann International Max- Planck Research School: Interface-Controlled Materials for Energy NISE-project Conversion Wi3879/1-1 NanoMatFutur 13N12972 NSF/Solar DMR

42 Semiconducting nanocomposites with Thank you for your attention tailored optical and electronic properties S. Wippermann Max-Planck-Institut für Eisenforschung, Düsseldorf (c) Peter Allen NanoMatFutur research group 13N12972 BMBF Meeting, Marl, 08/31/2016

O Sn2S6 ligands dissociate without any barrier on InAs(100) and (111) free surface

(bulk) µ Sn µ Sn (bulk) µ S µ S (bulk) µ Sn +2µ S = µ SnS2 µ N 1 2 µ N 2 (gas) µ H

reservoir provides particles plot phase diagram as function of ΔμIn, ΔμS, ΔμH =>

43 O Sn2S6 ligands dissociate without any barrier on InAs(100) and (111) free surface energy: G = E tot E ref i n i µ i µ In µ In (bulk) µ In + µ As = µ InAs µ As µ As (bulk) µ Sn µ Sn (bulk) µ S µ S (bulk) µ Sn +2µ S = µ SnS2 µ N 1 2 µ N 2 (gas) µ H 1 2 µ H 2 (gas) 2µ N +4µ H = µ N2 H 4 μ corresponds to free energy at which reservoir provides particles plot phase diagram as function of ΔμIn, ΔμS, ΔμH => relative thermodynamic stability of different structures at specific synthesis conditions 30

44 Effects of structural details on the band alignment LDOS in NP and in matrix matching: NP-matrix hybridization, long range interactions Sn106S227 InAs NP SnS matrix Isodensity plot: CB states of NP Sn88S171 Ssub InAs NP SnS matrix Isodensity plot: VB states of NP

45 Effects of structural details on the band alignment Difference of LDOS in NP and in matrix Sn106S227 SnS matrix InAs NP Isodensity plot: CB states of NP Sn88S171 Ssub SnS matrix InAs NP InAs NP SnS matrix Isodensity plot: VB states of NP

46 Electronic states of the defects in the NC-composite S-chains in the matrix Sn106S227 S atoms S chains Isodensity plot: states of S-chains As in the matrix, under-coordinated Sn Sn88S171 Ssub S matrix As in matrix Isodensity plot: As in matrix

Search for materials to harvest light

Search for materials to harvest light Solar nanocomposites with complementary charge extraction pathways for electrons and holes: Si embedded in ZnS S. Wippermann, M. Vörös, F. Gygi, A. Gali, G. Zimanyi, G. Galli NanoMatFutur DPG-2014, 04/03/2014